Real-time content, such as voice or video streams, is often being exchanged between subscribers over a communication network. The type of network (circuit switching, packet switching), available bandwidth and the transmission media (cable, wireless, satellite, etc) may vary from one network to another. Regardless of the network type and the transmission media, in order for the real-time content to successfully traverse the network without degradation in user experience, the network must present two key properties in regard to that content:                a. Sufficient capacity (bandwidth).        b. Jitter-free transmission path. Low jitter up to 20 milliseconds is often considered acceptable, as its affect on voice quality is negligible and most available equipment is capable of eliminating such low jitter (using a relatively short jitter buffer).        
If a communication network does not present one or both of the above properties, content may be lost or suffer significant degradation in quality, up to a level where it is no longer comprehendible by the receiving party. Therefore, if either or both of the above properties cannot be guaranteed upon session setup for as long as the session is to be maintained, it is often regarded a good practice to refuse the session rather than to attempt it under inappropriate conditions.